1. Field of the Invention
This invention relates to iron oxide pigments which have a low silking effect and to a process for the production thereof.
2. Description of the Prior Art
Iron oxide yellow (.alpha.-FeOOH) pigments may be produced by the precipitation of iron salt solutions and oxidation with air (U.S. Pat. No. 2,388,659); or with other oxidizing agents, optionally in the presence of metallic iron (DE-PS No. 902,163); or they may be produced by the oxidation of metallic iron with oxidizing agents, in particular with aromatic nitro compounds (DE-PS No. 517,758).
Such iron oxide yellow pigments are used in the building material, plastics and lacquer industries.
The particle size and the length:width ratio of the needle-shaped iron oxide particles are primarily responsible for the optical properties of the iron oxide yellow pigments. The pigments become brighter and purer in color as the particle length: width ratio increases.
In paintings or articles pigmented with coating compositions which contain distinctly needle-shaped yellow iron oxide pigments, i.e. those pigments which are bright and pure in color, clearly perceptible color differences become visible. These color differences, known as silking effect, are dependent on the direction of observation of the painting or pigmented article. The silking occur when the pigments become orientated in a privileged direction when they are processed into a lacquer or film, which direction is parallel to the axis of the needles. Such color differences, which depend on the direction of observation of the paintings or pigmented articles are known from multi-colored silk fabrics. Since the absorption and scattering coefficients of conventional needle-shaped iron oxide yellow pigments are different in the longitudinal direction than in the direction perpendicular to the needle axis, the silking effect and thus the color difference is greatest when the needles are first oriented perpendicularly and then parallel to the direction of observation of the pigmented articles. Disturbing color shades are observed when areas having different orientations of the needle-shaped pigments meet. They become visible at the corners of door frames or window frames if the needle-shaped pigments are oriented parallel to the direction of observation when paint is applied to the vertical edges, and if they are oriented perpendicularly to the direction of observation when paint is applied to the horizontal edges.
The so-called picture frame effect is particularly disturbing. This effect occurs when a painter brushes along an edge a second time as he finishes painting a relatively large surface, in order to even out the edge. Since this orients the needle-shaped pigments in one privileged direction at the edge of the surface, it gives the impression that the surface is framed with a different color.
It is possible to reduce the degree of silking of iron oxide yellow pigments by mixing needle-shaped iron oxide pigments with isometric yellow pigments, such as nickel rutile yellow. Isometric pigments do not naturally exhibit a silking effect.
Isometric iron oxide yellow pigments are of limited use because they are dark and unsaturated in color. However, other isometric yellow pigments are also of limited use because they are several times more expensive than iron oxide pigments.
Thus, an object of the present invention is to provide bright yellow iron oxide pigments which are pure in color and which are virtually free of the undesired "Silking" properties.
This object is achieved by means of highly-branched needle-shaped iron oxide yellow pigments which have such a high degree of branching that, optically, they behave almost like isometric pigments, and they combine the desired favorable color properties with a silking effect which is virtually no longer perceptible.